@joshuabonnett: "Hi, I am looking at buying a server with 6 36gb 15k rpm scsi disks in it 
and I was wondering what people would recommend for a disk setup. The 
six need to include the os and such. As it is right now we are thinking 
1 4 disk raid 1+0 volume for postgres and 1 raid 1 volume for os and the 
swap file. Getting a 7th disk is economically impractical. What do 
people think?  This is a fairly write heavy db compared to some so I was 
thinking it would be good to avoid raid 5 or am I wrong here? What do 
people think?
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    @andrew: "-BEGIN PGP SIGNED MESSAGE-
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Hi Josh,
How long is a piece of string?
You will probably find that Raid 1 is almost as fast as Raid 1+0  
depending on your application.
In our setup we have Raid 1 for the System volumes, and 1x Raid 1 for  
the Xlog, and 6x Raid 1 for the data.
We spread the data across multiple disks based on usage.
Andrew
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    @cas: "<quote>
write speed isn't so bad on RAID 5 as long as you have a hardware raid
controller with a large (128MB+) non-volatile write-cache.
but really, the only way to be sure is to test it with your exact hardware.
for testing, stick in a smallish boot drive (whatever you have spare)
and install debian on it. then reconfigure the raid disks in various
ways (and formatted using various filesystems - ext3, xfs, reiser,
whatever) and run bonnie++ on it each time.
should take about a day or so to run all the tests....but well worth it.
when you've found the best performance (or acceptable performance with
best size), remove the test boot drive and rebuild the machine in its
final configuration.
for a long-lived server, this is pretty much the only way to guarantee
getting the best performance. in theory, raid 1+0 should beat raid 5,
but practice has a way of trumping theory every time.
NOTE: non-volatile (i.e. battery-backup) cache is essential - it saves
your data in case of power failure or crash. most good scsi hardware
raid controllers have battery-backup for the cache as an option.
craig
craig sanders <email_address>           (part time cyborg)
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    @joshuabonnett: "Thanks for the tip on bonnie++ I hadn't heard of it.
<quote>
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    @andrew: "-BEGIN PGP SIGNED MESSAGE-
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<quote>
Bonnie is great for testing throughput but not really a great test for
every database.
Actually - to run proper tests will probably take much longer - but  
agreed,
it is definitely worth it.
RAID 1+0 is great if you have a lot of data which needs to be  
streamed, (full
table scans) but for Random access, it is not really better than a well
implemented Raid 1.
You will probably find that you are better off running two raid 1s  
and separating
your data across multiple disks than you are running one raid 1+0.
You may also want to look at putting your indexes on different  
physical disks as well.
We are currently 4x raid 1s for data, 1x raid 1 for indexes and 1x  
raid 1 for the xlog.
Andrew
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    @andrew: "-BEGIN PGP SIGNED MESSAGE-
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<quote>
Going back to the disk question - why don't you get yourself a 1HE  
Server, with 2x system disks (10K SCSI/SAS)
and buy an external JBOD/ RAID system where you can keep adding disks  
as the needs require - either SCSI, SAS or FC.
Infortrend sell a relatively price friendly 16 disk JBOD/ RAID shelves.
Regards
Andrew
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